Both tests work, but there are issues with glitching and tempo. Seems also that pretty small audio buffers are being used due to the sound of the glitches.

Possible future workarounds for JS's lack of realtime guarantees for low-latency apps were discussed recently: https://news.ycombinator.com/item?id=7906674

But I think for now the only solution is "use bigger buffers" (or an easy way for users to experiment with the latency/safety tradeoff knob)